Insulation materials are currently utilized in applications which include windows, buildings, appliances, as well as other industrial and home equipment. One type of insulation is "powder-in-vacuum" insulation, wherein a vacuum space is filled with a particulate insulating composition, such as a powder. Powder-in-vacuum insulation may be utilized in insulation panels, in applications including home and industrial appliances, including refrigerators, water heaters, and equipment for the storage and transport of high and low temperature liquids.
The terminology "vacuum panel" or "vacuum insulation panel" may be utilized to describe powder-in-vacuum insulation, e.g. an insulation structure having reduced internal pressure. Vacuum insulation panels generally comprise an insulating composition and a vessel formed of a substantially impermeable film surrounding the filler material. The vessel is generally evacuated to an internal pressure below atmospheric pressure and hermetically sealed. In addition to providing insulating value, the functions of the insulating composition include supporting the skin of the vessel so that the vessel does not collapse during evacuation.
Vacuum insulation panels may be advantageously utilized in insulation applications including the walls of the refrigeration appliances. It is advantageous for such panels to remain effective for the life of the appliance, which may exceed 20 years.
The insulating characteristics of a vacuum panel are a function of the thermal conductivity of the particulate composition utilized and the degree of vacuum established and maintained.
There are at least several difficulties which may be encountered in the use of an insulating system in which a vacuum space is filled with an insulation material. In general, greater insulating values are achieved at vacuum levels farther beneath ambient, or put another way, thermal conductivity increases with increasing absolute pressure. As a result, manufacturing costs are increased by the need to establish vacuum levels farther beneath ambient (low absolute pressures), to provide a greater initial insulating value (lower thermal conductivity) and to minimize the reduction of the insulating value as a result of the loss of vacuum level through leakage, temperature increase and/or insulation material off-gassing. In addition, manufacturing and production costs are increased by the need for substantially gas impermeable barrier materials (e.g. vacuum bags) which will hold the insulation material, and maintain low vacuum levels.
It would be advantageous to have compositions which achieve improved thermal conductivities. Such compositions would be advantageous for use in insulation structures including, but not limited to, insulation panels having reduced internal pressures or other forms of powder-in-vacuum insulation.